1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and is suitably applicable to image pickup apparatuses such as a digital video camera, a digital still camera, and a silver-halide-film camera.
2. Description of the Related Art
A large-aperture zoom lens in which a lens unit having a negative refractive power is provided nearest to an object-side end is advantageous in performance improvement. Therefore, various proposals concerning such a zoom lens have been provided. For example, U.S. Pat. No. 7,184,221 discloses a large-aperture zoom lens including, in order from an object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power.
Japanese Patent Laid-Open No. 11-295601 discloses a large-aperture zoom lens including, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. A zoom lens in which a first lens unit having a positive refractive power is provided nearest to the object-side end is advantageous in reducing the total lens length (a length from the lens surface nearest to the object-side end to an image plane) and the lens diameter.
A large-aperture zoom lens in which a lens unit having a negative refractive power is provided nearest to the object-side end tends to be large and heavy. A large-aperture zoom lens in which a lens unit having a positive refractive power is provided nearest to the object-side end is advantageous in size reduction, but is difficult to provide a sufficient back focal length if the angle of view is increased.
The zoom lens disclosed by U.S. Pat. No. 7,184,221 has an f-number as small as 2.8 and an angle of view as wide as about 84 degrees, but its lens system is of a large size.
The zoom lens disclosed by Japanese Patent Laid-Open No. 11-295601 has an f-number as small as 2.8 with its lens system being of a small size, but its angle of view at the wide-angle end is about 65 degrees at most. This zoom lens is desired to have a wider angle of view.
Moreover, variations in spherical aberration and astigmatism during zooming are not satisfactorily corrected. In addition, there remain variations in coma aberration. With such a lens configuration and such a refractive power arrangement, it is difficult to realize a zoom lens having a large aperture and an angle of view that is wider than 2ω=80 degrees.